1. Technical Field
In one general aspect, the present invention relates to a filter circuit.
In the art, there is a general need for filtering signals. Especially, in situations where a signal comprises a main frequency component and disturbing frequency components at frequencies differing from the main frequency component, there is a need for filtering away the disturbing frequency components. If the disturbing frequency components are to be expected both at higher frequencies and at lower frequencies, relative to the main frequency component, the filter to use is a bandpass filter. If the main frequency is known in advance, persons skilled in the art will know how to design a bandpass filter circuit having a passband centered around the expected main frequency component. Depending on the requirements put on the filter, the width of the passband may be designed to be very narrow. However, if the main frequency is not exactly known in advance, for instance because the main frequency may vary within a relatively wide range, a narrow passband is not useful.
In another aspect, the present invention relates to a device for detecting zero-crossings in a signal.
In the art, there is a need for a reliable device for detecting zero-crossings in a signal. Although this need is a general need in the field of electronics, this need exists especially in the field of optical data devices, and therefore the present invention will, hereinafter, be explained in more detail for this specific practical example. However, it is stressed that the present invention is not limited to this example, but can be applied in a broader sense.
2. Related Art
Data recording media for recording information thereon in a plurality of substantially parallel tracks, or having information recorded thereon in a plurality of substantially parallel tracks, are commonly known, one typical example being an optical recordable CD. Information is recorded on such medium or read from such medium by means of an optical pick-up means, comprising a laser device which directs a laser beam towards the medium. The laser spot on the medium needs to follow the tracks accurately. Control of the pick-up position is performed by means of a tracking servo system, in which an error signal is developed which is indicative for the radial distance between the center of the track and the actual position of the laser spot, and the position of the pick-up means is adapted such as to minimize the error signal. Such a tracking servo system is commonly known per se, and need not be explained in more detail here.
An important feature of optical (or magnetic for that matter) recording media is that it is possible to steer the pick-up means directly to a desired track, as in contrast to conventional mechanical recording media where the information was recorded in one continuous spiral-shaped groove which should be followed by a needle. In order to be able to displace the pick-up means towards a desired track, it is at least necessary to have information available regarding the actual radial position of the pick-up means with respect to the target position of the desired track. When a control unit receives a command for displacing the pick-up means towards a new target track, the present position (present track) of the pick-up means is known. Basically, therefore, displacing the pick-up means towards the desired target track is performed by calculating the number of tracks between the present track and the target track, and to displace the pick-up means in such a way that the calculated number of tracks are skipped. Herein, it is necessary to detect the crossing of the tracks.
For this purpose, the above-mentioned error signal is used. During displacement of the pick-up means, the error signal is monitored; each time the error signal crosses zero, this occurrence is indicative for the crossing of a track. Therefore, it will be clear that, in order to be able to count the number of track-crossings accurately and in a reliable way, it is necessary to be able to detect the zero-crossings in the monitored error signal in a reliable way.
Although the method of detecting track-crossing by means of detecting zero-crossing and the error signal is known per se, for instance from German Offenlegungsschrift 40.29.975, the accuracy of the method is affected by disturbances in the error signal, which may cause a valid zero-crossing to be missed or an incorrect zero-crossing to be detected, and consequently causing the number of counted track-crossings to be incorrect. The disturbances in the error signal can have several causes.
One possible source of disturbances in the error signal is the presence of scratches or dust on the surface of the disc.
Another important source of disturbances is found in recordable optical media according to the ISO Standard. Such recordable optical medium comprises a so-called xe2x80x9cpre-groovexe2x80x9d or xe2x80x9cpre-trackxe2x80x9d, subdivided into sectors where information can be recorded. Each recordable sector is preceded by a sequence of preformatted pits, indicated as the xe2x80x9cheaderxe2x80x9d, which includes the so-called xe2x80x9cmirror markxe2x80x9d, which is an interruption of the pre-groove. Depending on the speed and timing with which the spot crosses the mirror mark, a track may be missed or counted twice.
According to an important aspect of the present invention, the error signal is passed through a filter in order to eliminate disturbed signal components.
Generally, filter circuits can be divided into three categories: lowpass, highpass and bandpass.
Just using a simple low pass filter for filtering the error signal is insufficient if the access algorithm involves very high velocities. In that case, a fixed lowpass filter must accommodate the highest speed possible, and consequently the filtering at lower speed will be ineffective.
Apart from the above-indicated possible high-frequency disturbances, there are also potential sources for low-frequency disturbances, particularly disturbances that are synchronous with the disc rotation, such as reflection variations, radially orientated scratches, etc.
Another possible cause of disturbances in the error signal is an offset in the error signal, indicated as xe2x80x9cbaseline shiftxe2x80x9d, that temporarily shifts the error signal above or below its normal baseline, which makes the zero-crossing in the error signal very vulnerable to noise.
Using a lowpass filter does not solve the problem caused by the above-mentioned baseline shift; for solving this type of problems, highpass filtering of the error signal is required.
Therefore, according to a further aspect of the present invention, it is possible to overcome the two above-mentioned problems simultaneously by using a bandpass filter for filtering of the error signal, preferably a narrow bandpass filter, the center frequency of which corresponds to the expected frequency of track-crossing during access. In this respect, as will be clear to a person skilled in the art, the expression xe2x80x9cnarrowxe2x80x9d is to be understood as meaning just broad enough that that the expected track crossing frequency always lies within the passband, taking into account all tolerances and variations (component tolerances, track pitch variations, etc.).
In practice, the speed with which the pick-up means is displaced over the surface of the disc can vary widely. In a typical 3xc2xd inch optical data disc with seek times of 30-40 ms, the frequency range of track crossings is from practically 0 to about 200 kHz. Therefore, in a further important aspect of the invention, the center frequency of the bandpass filter is controllable.
In principle, the drive means for displacing the pick-up means over the surface of the optical disc are controlled by a processor, which, inter alia, controls the speed of the pick-up means. In principle, such processor can also be used to set the center frequency of the bandpass filter; however, such setting would only be a rather coarse setting, and would not be accurate enough. Further, if for some reason an error would occur, and the center frequency setting of the bandpass filter would be erroneous to such extent that the output of the filter would become too small to provide reliable information, the control algorithm might crash.
Therefore, in a further aspect of the present invention, the bandpass filter is preferably self-tracking, i.e. it comprises a control loop which controls the center frequency of the bandpass filter and automatically adapts this center frequency to the frequency of the input signal.